Oil is an essential lubricant in your engine. It lets metal press against metal without damage. For example, it lubricates the pistons as they move up and down in the cylinders. Without oil, the metal-on-metal friction creates so much heat that eventually the surfaces weld themselves together and the engine seizes. Which is not good if you’re trying to get somewhere. On the other hand, if you want someone else not to get somewhere, then draining the oil out of his or her engine is an effective roadblock!